It sounds like your hollyhocks are suffering from Powdery Mildew. In this case the leaves and stems become covered with grayish white powdery spots and patches. Generally the patches are on the tops of the leaves. The infected leaves eventually curl up, discolor and die.

Powdery Mildew is caused by a fungus that thrives in damp AND dry conditions. It is very critical that you give the plants good air circulation. You must also rake up and remove any fallen foliage as that will spread the disease. The best thing you can do is to buy resistant varieties of Hollyhocks. Ask for them specifically where you buy your plants.

Hollyhocks are not the only plants that get this disease; many others are also susceptible, though ususally different strains of the disease affect different plant families.

Sprayed onto the affected plants, ordinary baking soda provides an excellent control. To make this solution mix 1 tablespoon of baking soda and 2 1/2 tablespoons of ultra-fine horticultural oil with a gallon of water. The oil is slightly fungicidal and acts to help the baking soda coat the leaf and hang on longer. Apply as a spray as soon as the mildew appears and every 10-14 days thereafter. Be sure to coat all surfaces!